The rolling contact bearing arrangement according to the invention is mainly suitable for the dynamically stable support of shafts driven at high revolutions, for example in exhaust gas turbochargers the rotors of which rotate, for example, at up to 60,000 revolutions per minute. To achieve stable quiet running of the shaft which carries at its two ends an overhung turbine wheel or compressor wheel, and in order to avoid uncontrolled conditions of slip in the rolling contact bearings, which are usually angular contact ball bearings, the latter are axially pre-tensioned. For this purpose, helical or cup springs, arranged coaxially with respect to the shaft, by means of which the inner races of the bearings are clamped with respect to the outer races over the balls are used in most cases.
To maintain this pre-tension for any operating condition, it must not be too small which results in a not insignificant bearing dissipation. It impairs the life of the ball bearings and the accelerating up of the rotor with an increase in vehicle load in traveling operation.
In a bearing arrangement of a turbocharger rotor, described in German Offenlegungsschrift 3,531,313, which is mentioned there as belonging to the prior art, the two ball bearings are axially pre-tensioned by a helical spring which coaxially surrounds the bearing shaft and which is supported on two bushes which accommodate the ball bearings and which are floatingly seated with radial play in the hole in the bearing casing and which the shaft bearing is accommodated. In operation, the radial play between the said bushes and the hole accommodating them is taken up by the bearing lubricating oil which is under pressure, as a result of which a position of the shaft axle is to be achieved which has limited resiliency but is stable.
The fact that the two bushes accommodating the ball bearings are separated from one another and can therefore move independently of one another, has the disadvantage, according to the German Offenlegungsschrift mentioned, that the stability of the rotational movement and the position of the turbocharger shaft is not ensured to the desired extent. In addition, the axial pre-tensioning force in this embodiment is intended to be quite great, the consequence of a reduction in the bearing life and of the mechanical efficiency.
It is therefore proposed as a remedy in this document, to provide a single-part bush for the two ball bearings. A twisting of the bush is prevented by a pin which loosely engages the bush and the bearing casing and lubricating oil reaches the interior of the bush through plays between the two front faces of the bush and the bearing casing hole and through a hole in the bush pointing towards the turbine-side ball bearing, and vaporizes to become a lubricating oil mist which ensures reliable lubrication of the two ball bearings. The oil is also sprayed through the said hole against the more heat-stressed turbine-side ball bearing which, as a result, is also cooled.
Apart from this and from a more stable shaft position due to the single-part bush, however, the subjectmatter of this German Offenlegungsschrift also has the disadvantage of axial pre-tensioning of the two ball bearings which, just as in the case of the construction with two bushes initially mentioned, continuously acts in the same strength and also results in not inconsiderable bearing friction losses.